This invention relates to improvements in pulse combustion systems for heating air.
Pulse combustion heater systems have been known for many years. In such devices, a combustible fuel and combustion air are admitted into a pulse-combustion chamber where they are ignited to produce an internal explosion, with resultant generation of heat. Immediately after each such explosion, an accoustically-produced negative pressure in the chamber draws additional air and fule into the chamber through appropriate valves, whereupon the next explosion occurs and closes the valves until the next negative pressure occurs. Accordingly, once started, a self-perpetuating series of heat-releasing explosions are produced, with combustion air and fuel being sucked in automatically and intermittently through appropriate air and gas inlet valves as needed. In response to the combustion chamber pulses of high pressure, the hot exhaust gases from the combustion chamber are normally expelled forcefully through a tail pipe leading to an exhaust decoupling or expansion chamber, from which an exhaust-pipe line extends to an exhaust flue outlet.
In a typical system then, air from outside a building is drawn into the combustion chamber along with combustible gas to produce the desired pulse combustion therein, the exhaust gases resulting from this combustion then passing through the tail pipe, decoupling chamber, an exhaust-pipe line and a flue outlet to the exterior of the building; this flow occurs without requiring special fans or blowers, in response to the pumping action provided by the acoustically-resonant combustion chamber and tail pipe, in combination with combustion-air and fuel-gas inlet valves. Room air to be heated may be passed by forced draft over the exterior of the combustion chamber, the exhaust expansion chamber and at least part of the tail pipe, and then returned in heated condition to the room. The combustion chamber may in some cases be provided with external fins to enhance the heat exchanger operation. Normally it is also desirable, or necessary, to provide one or more mufflers in the exhaust pipe line to reduce the noise generated by the pulse combustion operation.
U.S. Pat. No. 2,916,032 of J. A. Kitchen, filed Oct. 10, 1957 and issued Dec. 8, 1959, shows one previously-known pulse combustion system for accomplishing the heating of room air. In the system of this patent, the pulse combustion chamber a is supplied with combustion air and fuel gas, the combustion chamber is provided with external heat transfer fins b, the hot exhaust gases from chamber a pass through an exhaust manifold c to a heat exchanger consisting of a plurality of tubes d in zig-zag form having heat transmitting metal plates e combined therewith, and the tubes and plates are arranged parallel with each other in a plane parallel with the direction of the flow of the air to be heated. The outlet ends of the heat exchanger tubes are connected to another gas manifold f, from which the exhaust gases are supplied to a compartment r serving as an exhaust gas cushion. The exhaust gases from compartment r are conveyed through a perforated pipe u to the atmosphere, this pipe serving as an exhaust gas muffler. The flow of the air to be heated is from beneath the exhaust manifold f, across the latter, then across the exterior surfaces of the heat exchanger, and finally across the exterior surfaces of the finned combustion chamber into the space to which the heated air is to be delivered.
While suitable for many purposes, systems such as that shown in the above-referenced patent generally require one or more special exhaust mufflers to minimize what would otherwise constitute very objectionably loud noise, caused by the explosions in the pulse combustion chamber. They are also subject to the problem that at least the inlet end of the heat exchanger operates at extremely high temperatures and must therefore consist of materials especially adapted for such high temperature operation, while its lower or outlet end operates at greatly reduced temperatures for which condensation of exhaust gases occurs, leading to the possibilities of metal corrosion near the outlet end of the heat exchanger. The heat exchanger in such a system is therefore such as to require, for best operation, use of materials which are not only satisfactory for extremely-high temperature operation, but are also of high thermal conductivity and resistant to corrosion by condensate formed in the lower temperature portions thereof. Ferrous metals have the necessary high-temperature stability, but have rather poor thermal conductivity and corrosion resistance; copper and aluminum, for example, on the other hand have excellent thermal conductivity and corrosion resistance but tend to soften at very high temperatures. Accordingly, in such a prior art system the simultaneous requirements of high thermal conductivity for best heat exchange, high-temperature stability in the face of the very high temperatures at the inlet of the secondary heat exchanger, and corrosion resistance near the outlet of the secondary heat exchanger produce problems with respect to the materials to be used in making the heat exchanger.
It is an object of the present invention to provide a new and useful pulse combustion system for the heating of air.
Another object is to provide such system which provides high thermal efficiency by providing a relatively large amount of heat-exchanger surface.
A further object is to provide such a system in which the need for special exhaust mufflers is greatly reduced or completely eliminated.
Still another object is to provide such a system in which the heat exchanger operates at relatively low maximum temperatures, whereby the materials used therein need only be compatible with requirements of corrosion resistance and high thermal conductivity, and are not also subject to the severe and often conflicting requirements of operation at very high temperatures.
It is also an object to provide such a system which is compact and relatively inexpensive to make and maintain.